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Repeat LASIK safer with calculations based on unoperated corneal thickness

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In planning for repeat LASIK, it is safer to make calculations based on the patient’s original unoperated corneal thickness rather than the corneal thickness at the time of enhancement, a study found.

Estimations of residual stromal depth based on the original corneal thickness measurement were “more conservative and thus safer” than estimations based on measurements before the enhancement procedure, J. Bradley Randleman, MD, and colleagues found.

They used two methods to calculate the residual stromal bed (RSB) thickness for 79 patients who had undergone primary LASIK and subsequently underwent a second or third LASIK procedure. The calculations were made using either the original corneal thickness minus flap thickness and all ablations (original CT method) or pre-enhancement corneal thickness minus flap thickness and enhancement ablation (repeat CT method). The main outcome was the difference in the RSB thickness after the second and third procedure as calculated by each method.

For 92% of eyes, the repeat CT method calculated a greater RSB thickness than the original CT method. For 54% of eyes, the RSB calculated with the repeat CT method was more than 20 µm greater.

“After the second LASIK, mean RSB thickness with the repeat CT method was 24 µm greater than with the original CT method (P < .0001),” the researchers reported in the January issue of Ophthalmology. “The tendency for a greater value with the repeat CT method was consistent over the range of RSB thickness.”

Most important, the authors said, was that in patients with estimated RSB of less than 250 µm by the original CT method, 50% of eyes after a second LASIK surgery and 73% of eyes after a third LASIK surgery had an estimated RSB thickness greater than 250 µm by the repeat CT method.

“This critical subset of patients may have undergone repeat LASIK based on one (repeat) method of RSB estimation and been advised against further surgery based on the other (original) method,” they noted.